Re: bug#31636: 27.0.50; lockfile syntax searchable from info manual

[Robert Pluim]

Noam Postavsky <address@hidden> writes:

> On 6 June 2018 at 03:43, Robert Pluim <address@hidden> wrote:
>
>>> And I think you'll want to give a limit
>>> to the looking-back calls, otherwise they can keep searching until the
>>> beginning of buffer.
>>
>> ((looking-back "(") should never look at more than one character.
>
> Perhaps it shouldn't, but Emacs doesn't make that kind of optimization:
>
> (with-temp-buffer
>   (let ((last-command-event ?a))
>     (dotimes (i 8)
>       (self-insert-command (expt 10 i))
>       (benchmark 1 '(looking-back "(")))))
> Elapsed time: 0.000036s
> Elapsed time: 0.000030s
> Elapsed time: 0.000066s
> Elapsed time: 0.000427s
> Elapsed time: 0.004068s
> Elapsed time: 0.040317s
> Elapsed time: 0.422173s
> Elapsed time: 1.514244s
>
> (with-temp-buffer
>   (let ((last-command-event ?a))
>     (dotimes (i 8)
>       (self-insert-command (expt 10 i))
>       (benchmark 1 '(looking-back "(" (1- (point)))))))
> Elapsed time: 0.000031s
> Elapsed time: 0.000027s [5 times]
> Elapsed time: 0.000041s
> Elapsed time: 0.000045s
>
>> I could always do (eq (char-before) ?() I suppose.
>
> That's probably better. But wait! This time you *should* have a backslash: ?\(

Thatʼs a matter of style, no? ?( works equally well, but emacs
convention appears to be ?\(

>> The regexp produced by (sentence-end) looks like itʼs fairly well
>> anchored. Besides, Iʼm not searching for an end-of-sentence, Iʼm
>> asking "Am I at end-of-sentence".
>
> Which is exactly why you should put a limit.

I donʼt understand that conclusion. If I have

some words.  some other words
             ^

with point at '^', (looking-back (sentence-end)) will match. If point is
not at an end of sentence, looking-back won't go backwards through the
buffer attempting to find one.

Although I could limit it to (point-at-bol 0) just to be extra super
sure, but my benchmarking shows no appreciable difference.

Robert